Hostname: page-component-7dd5485656-wlg5v Total loading time: 0 Render date: 2025-10-31T14:54:37.479Z Has data issue: false hasContentIssue false
  • English
  • Français

Melting of plagioclase in granite and related systems: composition of coexisting phases and kinetic observations

Published online by Cambridge University Press:  03 November 2011

Wilhelm Johannes  and
François Holtz
Get access Rights & Permissions [Opens in a new window]

Abstract

Melting experiments with plagioclases were performed in the systems Ab-An, Ab-An-H2O, Qz-Ab-An-H2O, and Qz-Or-Ab-An-H2O-CO2. The experimental products were analysed by electron microprobe, and the kinetics of the reactions were studied qualitatively.

Melting of the plagioclase in the system Ab-An (P = 1 atm, T = 1420°C) is very fast in the first minutes but becomes slower with increasing run duration and is incomplete even after 1000 hours. The Ab/An fractionation between melt and residual plagioclase is similar to that described by Bowen (1913).

Melting kinetics of plagioclase in the system Ab-An-H2O ( = 5 kbar, T = 1000°C) is controlled by the diffusion of water into the plagioclase structure. Melting is especially fast parallel to the a-axis. The experimental products show separation of melts and crystals.

In the tonalite system Qz-Ab-An-H2O, equilibrium melting could be observed down to 830°C ( = 2 kbar) but not a lower temperatures. The kinetics of the reaction is enhanced by deficiency or excess of alumina in the aluminosilicate melt surrounding the plagioclase crystals. The fractionation of Ab and An between melt and plagioclase crystals is more pronounced in the presence of quartz than in the Ab-An-H2O system. The ratio An/An + Ab is approximately 0·35 in the melt and 0·85 in the coexisting plagioclase T = 880°C).

In the haplogranodiorite system Qz-Or-Ab-An-H2O–CO2, melting reactions were performed at P = 0·5 kbar, T = 880°C, and of approximately 0·5. It is assumed that near equilibrium compositions of melt and coexisting residual plagioclase could be obtained in long duration runs (run time = 60 days). The distribution of Ab and An between melt and minerals is similar to that observed in the tonalite system. The partial melt coexisting with an An-rich plagioclase and Or-rich K-feldspar is relatively poor in An.

Keywords

granitemeltingplagioclasereaction kineticstonalite

Access options

Get access to the full version of this content by using one of the access options below. (Log in options will check for institutional or personal access. Content may require purchase if you do not have access.)

Article purchase

Temporarily unavailable

References

Bowen, N. L. 1913. The melting phenomena of the plagioclase feldspars. AM J SCI 35, 577–99.CrossRefGoogle Scholar
Chappell, B. W., White, A. J. R. & Wyborn, D. 1987. The importance of residual source material (restite) in granite petrogenesis. J PETROL 28, 1-2, 111–38.CrossRefGoogle Scholar
Chappell, B. W., White, A. J. R. & Williams, I. S. 1991. A transverse section through granites of the Lachlan Fold Belt. Second Hutton Symposium on Granites and Related Rocks, Excursion guide. REC BUR MINER RESOUR GEOL & GEOPHYS AUST, 1991/22.Google Scholar
Ebadi, A. & Johannes, W. 1991. Beginning of melting and composition of first melts in the system Qz-Ab-Or-H2O–CO2. CONTRIB MINERAL PETROL 106, 286–95.CrossRefGoogle Scholar
Gupta, L. N. & Johannes, W. 1982. Petrogenesis of a stromatic migmatite (Nelaug, Southern Norway). J PETROL 23, 548–67.CrossRefGoogle Scholar
Hine, R., Williams, I. R., Chappell, B. W. & White, A. J. R. 1978. Contrasts between I- and S-type granitoids of the Kosciusko Batholith. J GEOL SOC AUST 25, 219–34.CrossRefGoogle Scholar
Johannes, W. 1978. Melting of plagioclase in the system Ab-An-H2O and Qz-Ab-An-H2O at = 5 kbars, an equilibrium problem. CONTRIB MINERAL PETROL 66, 295–303.CrossRefGoogle Scholar
Johannes, W. 1980. Metastable melting in the granite system Qz-Or-Ab-An-H2O. CONTRIB MINERAL PETROL 72, 73–80.Google Scholar
Johannes, W. 1984. Beginning of melting in the granite system Qz-Or-Ab-An-H2O. CONTRIB MINERAL PETROL 86, 264–73.CrossRefGoogle Scholar
Johannes, W. 1985. The significance of experimental studies in the formation of migmatites. In Ashworth, J. R. (ed.) Migmatites, 36–85. Glasgow: Blackie and Son.CrossRefGoogle Scholar
Johannes, W. 1989. Melting of plagioclase-quartz assemblages at 2kbar water pressure. CONTRIB MINERAL PETROL 103, 270–6.Google Scholar
Johannes, W. & Gupta, N. L. 1982. Origin and evolution of a migmatite. CONTRIB MINERAL PETROL 79, 14–23.CrossRefGoogle Scholar
Kerrick, D. M. & Jacobs, G. K. 1981. A Modified Redlich-Kwong equation for H2O, CO2 and H2O-CO2 Mixtures at elevated pressures and temperatures. AM J SCI 281, 735–67.CrossRefGoogle Scholar
Koepke, J., Behrens, H. & Johannes, W. in prep. Melting kinetics of plagioclase dry and at 5 kbar water pressure. CONTRIB MINERAL PETROL (in prep.)Google Scholar
Mehnert, K. R. & Büsch, W. 1982. The initial stage of migmatite formation. N JB MINER ABH 145, 211–38.Google Scholar
Tsuchiyama, A. & Takahashi, E. 1983. Melting kinetics of plagioclase feldspar. CONTRIB MINERAL PETROL 84, 345–54.CrossRefGoogle Scholar
Yoder, H. S. 1968. Albite-anorthite-quartz-water at 5 kb. CARNEGIE INST WASHINGTON YEARB 66, 477–8.Google Scholar
Yoder, H. S., Stewart, D. B. & Smith, J. R. 1957. Feldspars. CARNEGIE INST WASHINGTON YEARB 56, 206–14.Google Scholar